Metformin as an anti-cancer agent against bladder cancer acts via PD-L1 downregulation in an orthotopic mouse model

Background PD-L1 is a crucial immune checkpoint protein that limits the effectiveness of antitumor immunity. Blocking the PD-L1/PD-1 pathway has shown promise in cancer treatment, but low response rates to checkpoint inhibitors highlight the need for alternative therapeutic strategies. Metformin, a drug primarily used for diabetes, has gained attention as a potential cancer treatment due to its effects on PD-L1 expression, the tumor microenvironment, and its ability to inhibit the proliferation of tumor cells. Meta-analyses suggest that metformin may reduce the incidence and improve the prognosis of bladder cancer in patients with type 2 diabetes, but the exact mechanisms by which it exerts its effects remain unclear. Materials This study employed a syngeneic orthotopic bladder cancer model with immunocompetent C57BL/6 mice to evaluate metformin’s therapeutic effects. Safe dosage was established through a maximum tolerated dose (MTD) test, identifying 150 mg/kg/day as suitable. Tumor size, body weight, survival, and PD-L1 expression were measured, while a tetrazolium-based assay assessed bladder cancer cell proliferation in vitro . Results Findings revealed elevated PD-L1 gene and protein expression in mouse bladder cancer tissues. In vitro , metformin inhibited PD-L1 expression and proliferation of MB49 mouse bladder cancer cells. In vivo , metformin reduced cancer cachexia, tumor size, and improved survival at 150 mg/kg/day. Importantly, metformin attenuated tumor-induced PD-L1 upregulation in a dose-dependent manner. Conclusion Overall, this study suggests that bladder tumors can increase PD-L1 expression to promote PD-L1–mediated intrinsic tumor growth pathways, and metformin effectively downregulates PD-L1 expression, suppresses bladder cancer cell proliferation, and prolongs survival in an orthotopic bladder cancer mouse model.